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Carrier-grade NAT (CGNAT)

published · September 12, 2026 · updated · October 2, 2026

Carrier-grade NAT puts many subscribers behind one public IP address, so one address is many people and a per-address decision reaches all of them.

Carrier-grade NAT is address sharing at the operator's scale: because there are not enough IPv4 addresses for every subscriber, a mobile carrier or a large ISP gives each customer a private address from the 100.64.0.0/10 block reserved for the purpose (RFC 6598) and translates it to one public address shared by hundreds or thousands of them. The public address a server sees is the carrier's gateway, and it changes as the handset moves.

What Spaw answers

Two things follow. The shared-address block itself, 100.64.0.0/10, answers reason: reserved_range and is free, because it can never be a public visitor. And a carrier's public gateway addresses are where is_mobile matters: the flag comes from a curated, positive-only list of mobile-carrier autonomous systems, and connection_type reads mobile for them. Their located city is the gateway's, so location_confidence answers low for them even where the carrier's own geofeed names it. Nothing in the answer can separate one subscriber behind that gateway from the next.

What it does not mean

A block or a rate limit keyed on a CGNAT address hits everyone behind it, which is a lot of innocent handsets. The velocity count in seen_by_you and any outcome you report are keyed by address too, so on a mobile network they describe the gateway, not the person; use them with that in mind, and prefer the signals that travel with the session rather than with the address.

Related

markdown version: /glossary/cgnat.md

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